The present invention generally relates to an improved cooling system and method for cooling the air in an enclosed space. The present invention more particularly relates to such a system which includes an improved fan control and method for controlling the speed and sequencing of a plurality of fans which cool the water used in a condenser water loop so that total power consumption of the system can be minimized.
Cooling systems, such as air conditioning systems, for cooling the air in an enclosed space, such as a building, are well-known in the art. Such systems generally include a condenser water loop having a cooling tower comprising a plurality of fans which provide cooled water to a condenser. Each fan is driven by a fan motor which consumes power. As the air flow provided by the fans is increased due to increased fan speed, an increased amount of heat is transferred from the water to the external air. Increased fan speed also results in increased fan motor power consumption.
Such systems further generally include a chiller which conducts a refrigerant through the condenser and an evaporator under the influence of a pressure differential provided by a compressor. The compressor includes a motor which consumes power and the power consumption of the compressor motor and hence, the chiller power, increases as the pressure differential is required to increase. An increased pressure differential is generally required when the temperature of the condenser water increases. Hence, through proper cooling tower fan control, the overall power consumption of the fan motors and the compressor can be minimized.
Lastly, cooling systems also generally include a chilled water loop. The chilled water loop conducts water through the chiller evaporator wherein heat is exchanged from the water of the chilled water loop to the refrigerant in the evaporator. The chilled water is also conducted through a set of coiled conduits. Air is blown over the coiled conduits to provide cooled air to a set of ducts which then distributes the cooled air throughout the enclosed space to be cooled.
In order to minimize the power consumption of such a cooling system, it is therefore necessary to properly control the speed of the cooling tower fans. One common control strategy is to vary the tower air flow in order to maintain a fixed water supply temperature to the chiller condenser. A better, but non-optimal, control strategy is to maintain a constant temperature difference between the cooling tower outlet and the ambient wet bulb.
By controlling cooling tower fans to set point values relating to variables that change only through time, such as to maintain a fixed approach to an ambient wet bulb temperature, these closed-loop control strategies are inherently unstable. Since cooling tower fans are customarily one- or two-speed fans requiring discrete control, the desired set point cannot be realized, resulting in an oscillating tower fan control.
Open-loop control strategies have also been suggested in the art. One such strategy utilized a control map tailored to a specific system which represented a look-up table for optimal control set points as a function of load and ambient wet bulb temperature. A practical difficulty resides in this approach because implementing this procedure centers on the method for determining the control maps. The optimal control maps were derived through the use of detailed computer simulations. This procedure is cumbersome, requiring considerable expertise, along with many measurements to match the models to the actual system performance.
The present invention overcomes the difficulties associated with the prior art approaches to cooling tower control. The system and method of the present invention utilizes open-loop control of the cooling tower fans and such open-loop control is performed by determining simple estimates of control parameters by utilizing fixed cooling system design information.